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Lithium Abundances and Rotational Behavior for Bright Giant Stars�, A&A 450, 1173–1179 (2006) Astronomy DOI: 10.1051/0004-6361:20053485 & c ESO 2006 Astrophysics Lithium abundances and rotational behavior for bright giant stars, A. Lèbre1,P.deLaverny2, J. D. do Nascimento Jr.3, and J. R. De Medeiros1,3 1 Groupe de Recherche en Astronomie et Astrophysique du Languedoc, UMR 5024/ISTEEM (CNRS), Université de Montpellier II, Place Bataillon, 34095 Montpellier, France e-mail: [email protected] 2 Observatoire de la Côte d’Azur, Département Cassiopée, UMR 6202 (CNRS), BP 4229, 06304 Nice, France 3 Departamento de Física, Universidade Federal do Rio Grande do Norte, 59072-970 Natal, RN, Brazil Received 20 May 2005 / Accepted 16 November 2005 ABSTRACT Aims. We study the links possibly existing between the lithium content of bright giant stars and their rotational velocity. Methods. We performed a spectral analysis of 145 bright giant stars (luminosity class II) spanning the spectral range from F3 to K5. All these stars have homogeneous rotational velocity measurements available in the literature. Results. For all the stars of the sample, we provide consistent lithium abundances (ALi), effective temperatures (Teff), projected rotational velocity (v sin i), mean metallicity ([Fe/H]), stellar mass, and an indication of the stellar multiplicity. The gradual decrease in lithium abundance with Teff is confirmed for bright giant stars, and it points to a dilution factor that is at least as significant as in giant stars. From the F to K spectral types, the ALi spans at least three orders of magnitude, reflecting the effects of stellar mass and evolution on dilution. Conclusions. We find that the behavior of ALi as a function of v sin i in bright giant stars presents the same trend as is observed in giants and subgiants: stars with high ALi are moderate or fast rotators, while stars with low ALi show a wide range of v sin i values. Key words. stars: abundances – stars: atmospheres – stars: evolution – stars: rotation 1. Introduction detection to the current interstellar medium and meteoritic ob- served value of about 3.2 dex. After a sudden decline around Lithium is a key element in the study of the chemical evolu- the referred spectral types, the abundances of lithium pro- tion of the Galaxy, as well as in understanding the evolutionary gressively decrease with effective temperatures once the stars history of stars, due particularly to its role as an important diag- evolve along the spectral types G to K. Qualitatively, such be- nostic of internal stellar mixing. In recent years, there has been havior reflects the dilution of surface lithium by convective a tremendous upsurge in the observational study of the stellar mixing as a consequence of the development of the convective ff lithium line at 6707.81 Å in field stars at di erent regions in the envelope toward the inner and hotter stellar regions, where Li Hertzsprung Russel diagram. The behavior of Li abundances as is expected to be destroyed rapidly by (p, α) reactions at tem- ff a function of e ective temperature is now well-established for peratures >2.4 × 106 K. Thus, subgiant and giant stars evolv- both subgiant stars (De Medeiros et al. 1997; Lèbre et al. 1999; ing into the G and K spectral types are expected to have low Randich et al. 2000; Mallik et al. 2003) and giant stars (Brown lithium abundances. Moreover, theoretical models predict that et al. 1989; Wallerstein et al. 1994; De Medeiros et al. 2000; surface lithium has to be diluted by a factor of 28 to 60 in stars de Laverny et al. 2003). of 1–9 M at the bottom of the red giant branch (Iben 1965, These studies have shown, in the broadest sense, two clear 1966a,b, 1967). distinct features in the distribution of Li abundances (ALi) along the spectral regions F, G, and K. Single subgiants and gi- After this stage, no additional convective dilution should ants bluer than the ones of the spectral types F8 and G0, re- occur. Nevertheless, the observational behavior of ALi shows that the dilution factor is far more important than the stan- spectively, present a wide dispersion in ALi from about null dard theoretical predictions, with values as high as 1000 within Based on observations collected at ESO, La Silla, Chile, and at the giant region (Brown et al. 1989; De Medeiros et al. 2000). the Observatoire de Haute Provence, France, operated by the Centre This fact points to additional mixing mechanisms operating in National de la Recherche Scientifique (CNRS). evolved stars. From different studies (e.g.: Brown et al. 1989; Table 1 is only available in electronic form at Fekel & Balachandran 1993; Drake et al. 2002) there is now a http://www.edpsciences.org growing list in the literature of single G- and K-type giant stars Article published by EDP Sciences and available at http://www.edpsciences.org/aa or http://dx.doi.org/10.1051/0004-6361:20053485 1174 A. Lèbre et al.: Li abundances and rotation for bright giants that violate this general rule, exhibiting ALi far in excess of the of rotational and radial velocities by De Medeiros & Mayor predicted values. The root-cause for such unexpected behavior (1999). Let us recall that this work compiles precise rotational is not understood clearly. velocity v sin i and signatures of either single or binary behav- In addition to the behavior of lithium versus effective tem- ior for a large and homogeneous sample of about 430 bright gi- perature, the relationship between ALi and the rotational ve- ants. The rotational velocities measured by these authors have − locity in subgiant and giant stars has been studied by differ- an uncertainty of about 2.0 km s 1 for stars with a v sin i lower − ent authors (Wallerstein et al. 1994; De Medeiros et al. 1997, than about 30.0 km s 1. For faster rotators, the estimations in- 2000; do Nascimento et al. 2000, 2003; de Laverny et al. dicate an uncertainty of about 10% on the measurements of 2003; Mallik et al. 2003). In spite of the fact that fast rota- v sin i. tors present enhanced lithium content, the connection rotation- Stars in our observational sample were selected to cover the ff lithium seems to be more complex and to depend on di er- spectral range from F3 to K5 in the luminosity class II. Such a ent stellar parameters. Slow rotators, e.g. typically giants and −1 selection ensures that we can focus on 3 regions: (i) the region subgiants with v sin i lower than about 10.0 km s ,showa where bright giants present a broad range in values of rotational wide range of ALi values with at least 3 mag of dispersion velocity (the F-type stars), (ii) the region of the discontinuity (De Medeiros et al. 2000; do Nascimento et al. 2003; de in rotation (the late F-type/early G-type stars), and (iii) the re- Laverny et al. 2003). Such dispersion apparently depends on gion of the slow rotators (the G- and K-type stars). Table 1 stellar mass (De Medeiros et al. 2000). lists the 145 program stars in order of increasing HD num- For stars of higher luminosity classes, namely for bright ber, with the respective projected rotational velocity v sin i from giants and supergiants, previous investigations of the behavior De Medeiros & Mayor (1999) and the stellar parameters de- of lithium and its connection to rotation have been hampered rived from the spectroscopic study (see next section). by the paucity of data. Luck & Wepfer (1995) have analyzed the behavior of ALi for a sample of 38 field bright giants of spectral types F, G and K. In spite of the limitation of their 2.2. Spectroscopic observations sample composed of 13 F-type, 22 G-type, and 3 K0 bright giants, these authors have observed, for this luminosity class, The data were acquired during three observational runs in the well-known gradual decrease in ALi with effective temper- August 1997, February 1998, and July 2003 for northern stars, ature established for other luminosity classes. Luck & Wepfer and in November 1997 for southern stars. We used three dif- (1995) have also found that Li depletions for bright giants are ferent telescopes and instrumentations to observe the spec- more severe than theoretical predictions with a dilution factor tral region around the lithium line at 6707.81 Å. For north- between 100 and 1000. ern stars, high-resolution spectra of the lithium region were With the main goal of settling the question of the be- acquired with the AURELIE spectrograph (Gillet et al. 1994) havior of lithium and its connection to rotation in evolved mounted at the 1.52 m telescope of the Observatoire de Haute intermediary-mass stars, we here present a spectroscopic study Provence (OHP, France). The spectrograph used a cooled of the lithium line at 6707.81 Å for a large sample of bright 2048-photodiode detector forming a 13 µm pixel linear array. giants with spectral types from middle F to middle K. For A grating with 1800 lines/mm was used, giving a mean dis- all these stars, precise radial and rotational velocity measure- persion of 4.7 Å/mm and a resolving power around 45 000 (at ments (vrad and v sin i) are available from De Medeiros & Mayor 6707 Å). For this instrument and the selected set-up, the spec- (1999). The combination of our spectroscopic data with these tral coverage was about 120 Å. The signal-to-noise ratio was v sin i measurements allows an unprecedented analysis of the always better than 50.
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